Segregation at stacking faults in a Co-Ni superalloy with the composition, Co-33Ni-20Cr-10Mo-xNb (wt%), with x = 0, 1, 2, was investigated by means of phase-field simulation in order to clarify the roles played by each element in the Suzuki effect; which was believed to be responsible for strain-aging. The simulation results suggested that Ni depletion could be very significant in the absence of Nb, and the deviation in concentration could exceed 4wt%. The stacking-fault energy decreased from 35 to -775mJ/m2 as a result of segregation. Niobium additions enhanced the Cr segregation significantly, and resulted in increases in stacking-fault energy although the strain-aging could be enhanced by Nb additions. The reasons for the discrepancy were explained from the viewpoint of the effect of Nb additions on the kinetics and energetics of Cr segregation.

Phase-Field Simulation of Suzuki Segregation in Co-Ni-Based Superalloy with Nb-Addition. Y.Koizumi, S.Suzuki, T.Otomo, S.Kurosu, Y.Li, H.Matsumoto, A.Chiba: Intermetallics, 2012, 27, 52-7